The present invention relates to a charged beam lithography method and apparatus for drawing a desired pattern, and more specifically to a method and apparatus for drawing a microscopic pattern such as a semiconductor integrated circuit pattern, etc., on a semiconductor substrate, such as a semiconductor wafer, a photo lithographic mask, etc., by use of a charged beam.
Conventionally, in order to form a circuit pattern on a substrate (e.g., semiconductor wafer, mask, etc.), there have been adopted a method of projecting a pattern in accordance with optical, projection lithography technique or a method of forming any desired pattern by use of a charged beam. In particular, the charged beam pattern drawing apparatus is mainly being used to fabricate, a photo lithographic mask used for the optical projection lithography. In this case, a pattern whose dimensions are four to five times larger than actual pattern dimensions on a wafer are drawn on the mask, under consideration of the reduction ratio of the optical projection lithography apparatus.
Recently, however, since there exists a tendency that the pattern dimensions decrease more and more with the advance of the device microminiaturization, the pattern dimensions are now approaching to the light resolution limit.
To overcome this problem, therefore, an ultra high resolution technique is now being put to practice use. In this technique, the resolution and depth of focus can be improved by improving the conventional photo lithographic mask (e.g., a phase shift mask) and the conventional optical projection system (e.g., modified illumination). In the case where a pattern is formed on wafer in accordance with the ultra high resolution technique, however, when a pattern formed on a mask is projected onto a wafer, there arises a problem in that such a optical proximity effect that the pattern image quality deteriorates becomes prominent. In order to reduce this optical proximity effect, a optical proximity effect correction technique has been proposed as disclosed in Japanese Patent Application Laid-open (Kokai) No. 58-200238 (1983). In this correction technique, as shown in FIG. 1, a plurality (e.g., four) of auxiliary patterns 2 to 5 having a predetermined shape smaller in dimension than a pattern 1 desired to be drawn on a mask (referred to as a desired pattern) are drawn at predetermined positions (e.g., corners or sides) relative to the desired pattern 1 together with the desired pattern 1. Here, the desired pattern 1 to be drawn on the photo lithographic mask implies a pattern enlarged at a predetermined magnification without deformation of a pattern 6 to be formed on a wafer. Further, this magnification is an inverse number of the reduction ratio of the optical projection apparatus.
Further, although the auxiliary patterns 2 to 5 are drawn on the mask in finite dimensions and shape, when the mask pattern is transcribed onto the wafer by the optical projection apparatus, the finite dimensions and shape are no more transcribed onto the wafer and therefore the auxiliary patterns are not actually formed on the wafer. Further, all the auxiliary patterns 2 to 5 are denoted by a comprehensive reference numeral 7 sometimes, hereinafter.
Further, in the following description, the desired pattern and the auxiliary pattern both to be formed on the photo lithographic mask are discriminated from each other in accordance with the above-mentioned definition.
Conventionally, in an electron beam pattern drawing apparatus for making a mask, an acceleration voltage of about 10 KV to 20 kV has been so far adopted. This is because a high resolution is not required for a pattern drawn on the photo lithographic mask, as compared with that required for the pattern formed on the wafer, and further the influence of the proximity effect is small for above acceleration voltage. Here, the scattering range (forward scattering) of the electron beam in resist (which exerts a direct influence upon the resolution) is about 0.1 to 0.2 .mu.m. Further, the resolution of the electron beam apparatus is designed at about 0.1 to 0.2 .mu.m. Therefore, for instance, when the desired pattern size on the wafer is reduced as small as about 0.25 .mu.m, so the pattern size on the photo mask of 1 .mu.m, the acceleration voltage and the beam resolution are both sufficient to draw a desired pattern on the photo lithographic mask. In this case, however, when the pattern dimension of a desired pattern is 1 .mu.m, since the dimension of the auxiliary pattern for compensating for the optical proximity effect is as fine as about 0.25 .mu.m (about 1/4 of the pattern dimension of the desired pattern) on the mask, there exists a problem in that the auxiliary pattern cannot be resolved on the mask on the basis of the above-mentioned acceleration voltage and the resolution.
In addition, in the conventional mask fabrication process, when the patterned resist is being developed or after having been developed (i.e., a light shading substance (e.g., Cr)) is etched, a dimension conversion difference is present. Therefore, in order to obtain a desired pattern dimension on the photo lithographic mask after the light shading substance has been etched, pattern drawing data are adjustably processed in such a way that the pattern width can be narrowed or widened under consideration of the dimension conversion difference, In this case, however, when there exist the auxiliary pattern for compensating for the optical proximity effect, since the pattern drawing data are adjustably processed so as to be narrowed under consideration of the dimension conversion difference, there exists a problem in that the auxiliary patterns cannot be formed or lost and thereby disappears.
In addition, in the case where the auxiliary patterns are added to mask pattern exposure data which includes the desired pattern, the number of figures in the mask pattern exposure data is increased. So, there exists a problem that a data amount of mask pattern exposure data is increased.
In addition, in the case where the auxiliary patterns are added to mask exposure data which includes the desired patterns, the number of the figures to be drawn by charged beam exposure apparatus is increased. So, there exists a problem that drawing process speed will become low.
The multi-pass-exposure is adopted to a charged beam exposure method in order to improve pattern positioning accuracy and pattern size accuracy.
In this case, since same patterns are drawn for two times or more than, the above problem becomes more serious by adding the auxiliary patterns to the mask pattern exposure data.